The present invention relates to a device for modulating a laser beam.
A method of marking the surface of a rolling mill roll by means of an intermittent laser beam is already known, in particular from the patent BE-A-870,609 now U.S. Pat. No. 4,329,562. The intermittent laser beam consists of successive pulses, emitted very close to one another, which strike the surface of the roll, the latter performing a rotational movement about its axis. Each pulse causes a very small zone of the surface to melt, thereby forming a relief consisting of a depression, or microscopic crater, surrounded by a partial or complete rim. Owing to rotation of the roll and a translatory movement of the laser beam with respect to the cylinder, this known method enables the formation of microscopic craters which are distributed in a substantially uniform manner on the surface of the roll, providing the said surface with a controlled, substantially isotropic and homogeneous roughness.
Moreover, a method for modulating a laser beam used notably for marking the surface of a rolling mill roll has been proposed, in particular in the patent BE-A-894,042 now U.S. Pat. No. 4,462,660. According to this method, in order to make the laser beam intermittent, use is made of a perforated rotating member revolving in the beam so as to successively interrupt and allow the beam to pass through. The direct beam, i.e. the beam passing through the perforated member and directly reaching the surface of the cylinder, strikes this surface in a first zone situated on the axis of the beam. Furthermore, the orifices allowing the laser beam to pass through the perforated member are provided with radial lateral faces which are inclined relative to the axis of the laser beam; thus, at least part of the beam is deviated, during interruption of the latter, towards another zone on the surface which is thereby subject to preheating or tempering, depending on the direction of rotation of the perforated rotating member.
However, this other zone on the surface does not generally coincide with the first aforementioned zone which receives the direct beam. The result is that this preheating has only little or no effect on the marking operation performed by the direct beam.
The object of the present invention is to provide a device for modulating a laser beam of the type mentioned above. The device according to the invention is designed in such a way that the energy of the reflected beam is used in an optimum manner so as to contribute towards the formation of the required degree of roughness.